Image display systems that employ image projection techniques, such as spatial light modulators (SLMs) using arrays of digital micromirror devices (DMDs), deformable mirrors, liquid crystal on silicon, and so forth, can suffer from a degraded contrast ratio (a ratio of brightest white to darkest black producible) due to reflections of a light used to project the images from parts of the arrays not intended to reflect light. Light can scatter from support structures, apertures, vias, and so forth from the array of light modulators. The scattered light can effectively reduce the darkness of the darkest black, therefore reducing the contrast ratio. For example, in a SLM using DMD technology, light can scatter from electrode structures, micromirror support structures, mirror vias, and so on.
One technique to reduce reflection is to place an antireflective coating on metal structures in the array of light modulators. Since the substrates and underlying structures can be a significant source of unintended reflection, the use of the antireflective coating can significantly improve the contrast ratio. For example, uncoated aluminum can have a reflectivity as high as 92 percent while coated aluminum can have a reflectivity of approximately two to three percent.
A second technique that can be used to reduce unintended reflection is to place a coating, such as an antireflective coating on the substrate of the integrated circuit. The coating can cover the substrate as well as any underlying structure formed in the substrate and help to reduce light reflectivity.
One disadvantage of the prior art is that the application of the antireflective coating on the substrate and the underlying structures as well as the metal structures is that the antireflective coating will have different reflectivity characteristics depending upon a wavelength of the light incident on the antireflective coating. Therefore, it can be difficult to create a single antireflective coating that can perform well with different wavelengths of light. This may lead to the use of an antireflective coating that is a compromise and result in a solution that does not yield optimal performance.
A second disadvantage of the prior art is that since the reflectivity characteristics of the antireflective coating can change depending upon the wavelength of the incident light, it may be possible that the use of the antireflective coating can have an undesired change in the color point of the display system. This can be a result of certain wavelengths of light reflecting in greater proportions than other wavelengths of light.
Yet another disadvantage of the prior art is that the antireflective layer can have an issue with non-uniform thickness. The non-uniform thickness may result in non-uniform antireflective properties.